1. Field of Invention
The present invention relates to a method for producing textured thermoplastic film and to the textured thermoplastic film produced thereby.
2. Description of the Prior Art
Textured sheet material has been widely applied in many fields. One particularly large scale use of textured sheet material is that of decorative coverings such as those used on ceiling and wall construction materials. Textured film is also increasingly being applied in the packaging field, for example, as protective overwraps for delicate articles such as electronic assemblies, glass, etc. Furthermore, textured film sheeting is used as a release mechanism in the production of rubber and associated materials. It is desirable that the film have suitable characteristics in order to fulfill requirements dictated by the end use. In many cases, it is important that the textured film have sufficient protuberance height in order to provide cushion properties, particularly when used for protective overwrap packaging. Additionally, many uses of textured film, for example, ceiling construction materials, require a pattern of random design. Textured sheet materials must meet minimum physical specifications, such as tensile strength, impact strength, and secant modulus, so that the films can be handled on fabricating machines.
Historically, textured films such as polyethylene, polypropolene, polybutene-1, and other films have been produced by various methods. One method used to produce textured film is to mechanically emboss a textured impression into the film. An embossing process used heretofore to produce a textured film comprising passing the film over a heated roll or series of heated rollers in order to heat the film to a softened state, contacting the film with an embossing roller; and pressing the film against the embossing roller with a back-up roller which has a resilient covering. Normally, the embossing roller and the back-up roller are cooled in order to freeze the textured pattern engraved on the embossing roll into the film so that it can be immediately wound onto rollers. An apparatus and process for preparing a textured embossed film according to the foregoing is shown in U.S. Pat. No. 3,246,365.
A major drawback of using an embossing process to produce textured film is the limitation of texture characteristics, such as protuberance height, size, spacing, etc. In order to alter the characteristics of the textured pattern, it is necessary to replace the embossing roll with a different embossing roll which bears the altered pattern characteristics. Furthermore, when embossing rolls which bear high protuberances are used for producing textured film, it is normally necessary to replace the resilient covered roll after a relatively short period of operation because protuberances of substantial height in the embossing roll quickly and severely wear the resilient covering. The worn resilient covered roll eventually lacks the smoothness needed to obtain adequate surface contact across the face of the embossing roll and therefore will not produce an acceptable textured film product. Obviously, the replacement costs adversely affect the economics of this method.
U.S. Pat. No. 3,180,775, issued Apr. 27, 1965, discloses a method of making non-woven fabrics by creating webs of shrinkable and non-shrinkable fibers which are bonded together and then subjected to shrinking conditions (including heat). This causes the non-shrinkable fibers to become puffed and distended and form ripples in the surface of the fabric. While this disclosure is somewhat similar to the present invention, there is no mention of using materials with dissimilar softening points or relieving orientation in one of the materials but not the other by heating both to a temperature above the softening point of one and below the softening point of the other.
Therefore, it can be seen that the textured film art is in need of the process of the present invention whereby preformed film, composed of two or more components possessing dissimilar softening points, can be heated to a sufficient temperature and cooled to obtain permanent protuberances in the film and yet which will permit a wide variation in the composition of the film and will utilize operating parameters which will provide a wide variation in protuberances such as height, size, spacing and general form.
It is an object of the present invention to provide a process for producing textured film which permits a wide variation in the character of the protuberances in the film.
It is a further object of the present invention to provide a method for producing textured film from a sheet film produced using the blown bubble process.
It is a still further object of the present invention to produce a textured film possessing desirable physical properties.